A substantial portion of the time required for drilling operations, completion operations, and/or fracturing operations is consumed in replacing worn metal pieces and/or equipment used for these tasks. Excessively high torque and drag may cause excessively costly delays or interruptions during downhole operations. The metal surfaces also wear down due to frictional forces, resulting in reduced equipment life. These problems generally increase at high temperatures and/or high pressures.
Lubricants or lubricating agents may be added to a downhole fluid to reduce or decrease friction, torque, and/or drag between two surfaces. This may be especially important when one or both surfaces are metal surfaces, such as within a metal-metal motor. Drilling fluids, completion fluids, and fracturing fluids will be discussed in more detail.
Drilling fluids are typically classified according to their base fluid. In water-based fluids, solid particles are suspended in a continuous phase consisting of water or brine. Oil can be emulsified in the water which is the continuous phase. “Water-based fluid” is used herein to include fluids having an aqueous continuous phase where the aqueous continuous phase can be all water or brine, an oil-in-water emulsion, or an oil-in-brine emulsion. Brine-based fluids, of course are water-based fluids, in which the aqueous component is brine.
Oil-based fluids are the opposite or inverse of water-based fluids. “Oil-based fluid” is used herein to include fluids having a non-aqueous continuous phase where the non-aqueous continuous phase is all oil, a non-aqueous fluid, a water-in-oil emulsion, a water-in-non-aqueous emulsion, a brine-in-oil emulsion, or a brine-in-non-aqueous emulsion. In oil-based fluids, solid particles are suspended in a continuous phase consisting of oil or another non-aqueous fluid. Water or brine can be emulsified in the oil; therefore, the oil is the continuous phase. In oil-based fluids, the oil may consist of any oil or water-immiscible fluid that may include, but is not limited to, diesel, mineral oil, esters, refinery cuts and blends, or alpha-olefins. Oil-based fluid as defined herein may also include synthetic-based fluids or muds (SBMs), which are synthetically produced rather than refined from naturally-occurring materials. Synthetic-based fluids often include, but are not necessarily limited to, olefin oligomers of ethylene, esters made from vegetable fatty acids and alcohols, ethers and polyethers made from alcohols and polyalcohols, paraffinic, or aromatic, hydrocarbons alkyl benzenes, terpenes and other natural products and mixtures of these types.
There are a variety of functions and characteristics that are expected of completion fluids. The completion fluid may be placed in a well to facilitate final operations prior to initiation of production. Completion fluids are typically brines, such as chlorides, bromides, formates, but may be any non-damaging fluid having proper density and flow characteristics. Suitable salts for forming the brines include, but are not necessarily limited to, sodium chloride, calcium chloride, zinc chloride, potassium chloride, potassium bromide, sodium bromide, calcium bromide, zinc bromide, sodium formate, potassium formate, ammonium formate, cesium formate, and mixtures thereof.
Chemical compatibility of the completion fluid with the reservoir formation and fluid is key. Chemical additives, such as polymers and surfactants are known in the art for being introduced to the brines used in well servicing fluids for various reasons that include, but are not limited to, increasing viscosity, and increasing the density of the brine. Water-thickening polymers serve to increase the viscosity of the brines and thus retard the migration of the brines into the formation and lift drilled solids from the well-bore. A regular drilling fluid is usually not compatible for completion operations because of its solid content, pH, and ionic composition.
Completion fluids also help place certain completion-related equipment, such as gravel packs, without damaging the producing subterranean formation zones. The completion fluid should be chemically compatible with the subterranean reservoir formation and its fluids.
A fracturing fluid is injected into a well as part of a stimulation operation. Fracturing fluids may include water, proppant, and a small amount of nonaqueous fluids designed to reduce friction pressure while pumping the fluid into the wellbore. Such fluids often include gels, friction reducers, crosslinkers, breakers to reduce the viscosity of the gel, and surfactants. The type of additive added to the fracturing fluid is selected depending on the needs for improving the stimulation operation and the productivity of the well.
A drill-in fluid may be used exclusively for drilling through the reservoir section of a wellbore successfully, which may be a long, horizontal drainhole. The drill-in fluid may minimize damage and maximize production of exposed zones, and/or facilitate any necessary well completion. A drill-in fluid may be a brine-based fluid that contains solids having appropriate particle sizes (salt crystals or calcium carbonate) and polymers. Filtration control additives and additives for carrying cutting may be added to a drill-in fluid.
A workover fluid is a fluid for repairing or stimulating an existing production well for the purpose of restoring, prolonging or enhancing the production of hydrocarbons.
It would be desirable if the lubricants added to downhole fluids could better lubricate a first surface for reducing friction, torque, and/or drag when the surface contacts a second surface, particularly when one or both of the surfaces is metal.